The operating method of a data storage device includes storing data in a buffer memory according to an external request, and determining whether the data stored in the buffer memory is data accompanying a buffer program operation of a memory cell array. When the data stored in the buffer memory is data accompanying the buffer program operation, the method further includes determining whether a main program operation on the memory cell array is required, and when a main program operation on the memory cell array is required, determining a program pattern of the main program operation on the memory cell array. The method further includes issuing a set of commands for the main program operation on the memory cell array to a multi-bit memory device based on the determined program pattern.